In the use of pressurized air for various purposes such as industrial, experimental or medical purposes, it is common practice to remove liquid and solid contaminants prior to the utilization of the compressed air. One common contaminant in compressed air is water vapor, which since it can condense to liquid water due to a temperature drop, is customarily removed by a suitable desiccant system as is well known in the art. A particular disadvantage of desiccant systems is that desiccants degrade such as by attrition during use and introduce solid particulate matter (desiccant fines) into the stream of compressed air passing therethrough. Silica gel, activated alumina, and molecular sieves are commonly used desiccants in pressurized air systems and the particulate matter generated by such materials is in the overall size range of 200 to 1 microns with a preponderance of the particles being less than 20 microns.
Prior art filters for the applications intended for this invention are based on one of two filtration techniques: surface filtration; in-depth fibrous bed filtration. Each of these types of filters, although they can be designed to be highly efficient, have a propensity for clogging and a relatively short service life. To retain their efficiency, the only means utilized to extend service life is to increase their physical size and the quantity of filter media utilized which is usually not economically feasible or does not make for a practically compact design.
Surface type filters operate on the principle that a dust layer collects on the surface of the filter medium and the dust layer itself becomes the effective filter medium. The pores in the surface filter media are usually many times the size of the dust particles so that collection efficiency is low until this dust layer is built up. Ordinarily in this type of filter, design must be such that pressure drop across the built-up dust layer is limited, which limits the depth of this dust bed build-up and the flow rate of the gaseous fluid. If the pressure drop across this dust bed is excessive, bed rupture can occur and result in a severe loss of filtration efficiency. Also, during initial startup and before sufficient particles have been collected to build up a "precoat" in the surface filter pores, filtration efficiency is very low. If the pore size is initially made small enough to restrict the passage of the smaller particles to overcome this initially low filtration efficiency, extremely rapid clogging occurs before bed build-up can occur.
In-depth type filters consisting of a fibrous bed operate on the principle of the fibers collecting the dust particles by three basic mechanisms: impingement, i.e., the particle impacts the fiber; interception, i.e., the particle grazes the fiber and adheres to it; diffusion, i.e., the particle, because of its small size and random movement independent of the air path, deposits on the fiber. Such filters can be designed to be highly efficient even for submicronic particles and do contain considerable void volume. However, since all of the full size range of particles are collected within the filter media, including coarser particles, such filters can become clogged because of their finite volume to retain particles or contain a non-uniform distribution of solid particles resulting in air "channeling" and result in a loss of efficiency. If the in-depth type filter is designed to be efficient for the collection of very small particles, this requires very fine bed fibers. When the gas contains a broad range of size of particles, the larger size particles are all collected near the surface of the filter bed and can "choke" the fine fibers leading to high pressure drop, loss of stability of the bed, and by the resulting compression of the bed decrease the filtering capability of the underlying fibers.